PULLIN: You have already touched on the problem of compatibility between crosses of
different species. What do you think about the technique of premaxilla clipping to reduce
male aggression in confined environments?
LOVSHIN: In a confined environment, male aggression can be a problem, but we have
never had any such problem in a pond environment which allows the females to escape
from aggression. We never had any mortality from male aggression in the ponds we used,
which were about 350 m2 but when we brought fish into more confined environments,
3 to 4 m2, the problem did occur. During the time I was in Brazil, I did not know about
Lee's technique of cutting off the premaxilla, but at Auburn University, it works fine
and we have been using it quite a bit. The male fish is still aggressive. It still chases the
female but, because the premaxilla has been removed, he cannot wound her as easily.
Sometimes he will still run her around so much that I think she could die of exhaustion,
but he cannot break the skin.
JALABERT: What evidence do you have for the purity of the males which you used?
LOVSHIN: The only evidence I have is that we consistently got 100% all-male progeny.
We have been doing this in Brazil now for seven or eight years.
JALABERT: In group spawning or single pair spawning?LOVSHIN: Group spawning in Brazil, but 100% males have been obtained from single
pair spawning using stocks which we sent to Israel. Also, our stocks have produced 100%
males in Panama, in the United States and in Puerto Rico so I am pretty certain of the
purity of the strains.HENDERSON: You mentioned the problems of the reduced fry production with the
hybrid crosses. Do you think this is a problem of reduced fecundity or incompatibility?
LOVSHIN: I don't think it is basically fecundity, that is an egg production problem. The
number of eggs produced by actual spawning females is not abnormally low. I am con-
vinced that the problem is compatibility. There are certain females that will spawn
readily and repeatedly both in aquaria and tanks in normal pure species crosses. Now the
question is, can you select for this, i.e., take those females out, mate them with a normal
male, take the offspring, test them, select good females again, etc. Can we select genetic-
ally for good spawning in pure species crosses and for compatibility in hybrid crosses?
These are questions for the geneticists.
GUERRERO: Is there any evidence of hybrid vigor when you compare the growth
performance of hybrids with pure species progeny?
LOVSHIN: A good question and the answer is not clear from the literature because most
hybridization has been to produce all-males from pure stock male and female. Com-
parisons between the growth of an all-male population and mixed sex or female popula-
tions are complicated because we know that males grow faster than females-both hybrid
and pure species males. Where we compared all-male hybrids with the males of the other
species, we found that the differences in growth were not very great. In Brazil, we found
a difference, although I am talking here of a 10% to 15% advantage for the all-male
hybrid at best and sometimes this was not statistically significant. In Israel, we found
several cases where there was no difference at all.
ROBERTS: At the Institute of Aquaculture in Stirling, we have been producing very
pure lines using normal and sex-reversed (hormone-treated) sibling crosses. We found a
very significant growth promotion effect from the anabolic steroid treatment used for
sex reversal. More important, by crossing pure lines, we can produce hybrid all-male
progeny which, after 6 months or so, are very much larger than the pure lines. The
performance of these hybrids cannot be due to any residual effects of anabolic steroids.